This invention relates to radiation imaging systems and, more particularly, to an imaging system employing a radiation scanner with an array of infrared detectors in combination with signal conditioning circuitry employing AC (Alternating Current) coupled amplifiers wherein each amplifier is provided with a feedback circuit and a limiter operated by the feedback circuit to reduce detector voltage excursion of large targets, thereby to enhance the visibility of relatively small targets in the presence of the large targets.
One well-known form of infrared imaging systems employs an array of infrared detectors and a lens for directing rays of radiation from a portion of a scene being viewed by the array of detectors. Typically, the detectors are arranged along a line and are scanned in a direction perpendicular to the line, the line of detectors providing the width of a swath being swept during a single line of scan.
In one application of considerable interest, the scene being imaged is viewed on a display wherein the scan lines are parallel to a horizontal axis of the display while the line of detectors are parallel to a vertical axis of the display. Typical subject matter of the foregoing scene would include both sky and land. As viewed by infrared detectors, the sky presents subject matter which is of a lower temperature than the subject matter presented by the land. Also included within the typical scene would be a subject of higher temperature, such as aircraft within the sky, and cooler subject matter such as a lake or river on the land. Buildings, roadways, trees and other foilage may also be present as further subject matter within the scene.
Subject matter in the foregoing portions of the scene emit infrared radiation at differing temperatures characteristic of the subject matter in response to illumination by the sun, or in response to some other source of heat. Each of the foregoing detectors produces an electric signal of varying amplitude dependent upon the temperature of the subject matter of the portion of the scene being viewed by the detector. The signals produced by the array of detectors are processed in individual detector channels and may be applied to a display for presentation of an image of the various objects in the scene being viewed.
In viewing the foregoing scene, the scanner is often oriented relative to the scene such that the line of scan would be parallel to the horizon. As a result, individual swath of the scene swept by corresponding ones of the detectors would be predominantly cool or predominantly warm depending on whether a detector was scanning a portion of the sky or a portion of the land. A swath which traversed a variety of subject matter, such as a building and foliage, or a building and a lake, would present an average temperature between that of the sky and that of the land.
To the construction of infrared imaging systems, it is common practice to employ AC coupling in each of the detector channels. Such coupling may be introduced by the use of a capacitively coupled amplifier in each channel. The AC coupling blocks the portion of a detector signal which is produced by background radiation and, thereby, prevents an overdriving of the amplifier. The exclusion of the background signal is advantageous in that such background signal carries no useful information with regard to an outline of a subject in the scene being imaged. Also, the elimination of the background signal permits the full dynamic range of the amplifier and any signal processing circuitry which may follow the amplifier to be applied to the subject matter of interest.
A problem arises in that, within the portion of detector signal carrying useful information about the subject matter, there may be a large range of signal dynamics. For example, a scanned receiving beam passing by a stone building heated by incident solar rays, and situated in a grassland, would produce a large infrared signal. The large infrared signal would be detected by a detector and applied to a capacitor of the capacitively coupled amplifier. Upon being coupled by the capacitor, the signal would be distored by a droop due to a partial charging of the capacitor which charging would also introduce a DC (Direct Current) level shift to the capacitor. Both effects are detrimental to a viewing of line detail in the subject, such as windows in the building. Thus, it is seen that the capacitor, while providing a benefit in eliminating background radiation, also introduces a problem in the viewing of fine detail due to the partial charging of the capacitor by large signals in the subject matter.